Despite longstanding recommendations to vaccinate, preventable infectious diseases are still a major cause of disability and death in the United States. The current immunization schedule saves 33,000 lives, prevents 14 million cases of disease, and saves tens of billions of dollars in health care costs a year. While progress has been achieved, still an estimated 42,000 adults and 300 children in the United States die each year from vaccine-preventable diseases.

In an interview with 24/7 Wall St., Shannon Stokley, associate director for science at the Immunization Services Division of the Centers for Disease Control and Prevention, said that all vaccinations are important. In the United States, vaccines are used to prevent as many as 24 diseases in children and adults, from measles and polio to pertussis and the flu.

According to Stokley, the fourth dose of the DTaP vaccine is a good indicator of immunization status in a state. The DTaP is a series of vaccinations against diphtheria, tetanus, toxoids, and acellular pertussis, which is recommended for children in the second year of life. The fourth dose prevents against pertussis, known as whooping cough — a highly contagious disease that can be fatal in infants.

Cases of whooping cough have declined from around 200,000 cases a year before vaccination was introduced to around 1,000 at the low. In 2014, nearly 33,000 cases were reported.

To compare state vaccination coverage, 24/7 Wall St. reviewed the percentage of children 15 to 35 months old who have received the recommended dosage of this particular vaccination series. Parents are the least likely to have their children vaccinated in Wyoming, where 72.8% of young children have received the DTaP immunizations. Maine leads the nation with a vaccination rate among children of 93.1%.

Numerous factors explain the differences in vaccination coverage between states. State laws, prevention programs, and access to medical care all play partial roles.

Health insurance seems to affect immunization coverage to some degree. Most states with higher-than-average vaccination coverage rates also have higher insured rates. The percentage of adults without health insurance exceeds the national average share of 11.7% in only five of the 25 states with higher-than-average vaccination coverage rates. In 16 of the 25 states with low vaccination rates, the percentage of people without health insurance is above average.

The costs of vaccines and the lack of health insurance certainly prevents some families from immunizing their children. However, Stokely cited numerous programs providing free immunization services. Therefore educating and making parents aware of these programs is likely one of the best ways to increase immunization coverage in communities. “There are resources available for children so that cost should not be a barrier to getting a vaccine,” she said.

Whether state residents actually visit a doctor is of even greater importance. In a CDC study from 2014, researchers found that vaccination coverage tends to be higher among those with more physician contacts compared with those who had not visited a physician in the past year, regardless of health insurance status.

Because adults are less likely to go to the doctor than young children, Stokley noted, vaccination rates among adults are far lower than for infants. Vaccination rates increased during the 2012-2013 flu season. Still, only about 41.5% of U.S. adults are vaccinated against influenza.

To identify the states with the highest (and lowest) vaccination rates, 24/7 Wall St. reviewed in each state the percentage of 15-35 months old children who have received the 4+DTap vaccination in 2013- 2014 from the National Immunization Survey, a program produced by the Centers for Disease Control and Prevention. The 4+DTaP series includes the following vaccinations: diphtheria, tetanus, toxoids, and acellular pertussis. In addition, we considered the percentage of adults in each state who received the influenza vaccination in the previous year as of 2014 from the National Center for Chronic Disease Prevention and Health Promotion, Division of Population Health, also of the CDC. The percentage of people without health insurance came from the U.S. Census Bureau’s 2014 American Community Survey.